In many optical inspection or imaging tasks a well-defined illumination of the area to be inspected or imaged is required. For many such illumination purposes ring lights are used, for example in microscopy, where they are a common means to provide a dark field illumination. In such applications, it is desirable to restrict the area illuminated to the area of interest, and to have a homogeneous distribution of light within the area of interest. Possible light sources for example are arc lamps, LEDs (light emitting diodes), laser diodes, and halogen bulbs. While arc lamps typically provide higher light intensities than LEDs, they also exhibit stronger intensity fluctuations and shorter life times than LEDs; thus generally LEDs are a preferred choice of light source. As light from typical LEDs is emitted into a hemisphere around the LED, optical elements are required to direct as much as possible of the emitted light from one or plural LEDs into the area of interest, i.e. into the area to be illuminated.
The European Patent Application EP 1 919 001 A1 relates to a spot light device for product inspection, wherein an LED is used as light source. In order to homogenize the distribution of light across a certain area, the light from the LED is passed through a rod lens. The light from the LED is introduced into the rod lens by a condensing lens. In order to assure proper alignment of rod lens and condensing lens, and also in order to reduce the number of individual parts to be handled during assembly of an optical system, the rod lens and condensing lens are provided as sections of an optical unit, the rod lens constituting a light transmitting section and the condensing lens a light condensing section. The condensing section combines refraction and reflection in order to direct light from the light source into the transmitting section.
The European Patent Application EP 2 177 816 A2 discloses an array of light sources, in particular LEDs, the light of which is directed into a light integrator shaped as a rod. The light integrator homogenizes and constrains the light, based on reflection of the light within the integrator. The light integrator may be a hollow tube with reflective inner surface or a solid rod of an optically transparent material, where the reflection of light within the light integrator is due to total internal reflection. The cross-section of the light integrator may be circular, polygonal, or irregular. Further optical elements may be provided downstream from the light integrator. To each of the light sources there may correspond an optical element for controlling and directing the light from the light source. The light integrator may be tapered, in order to influence the divergence of the light exiting from the light integrator.
The European Patent Application EP 1 150 154 A1 discloses an illumination system, in particular for microscopes, wherein plural light sources, preferentially LEDs, are arranged in an annular carrier. The LEDs may be controlled individually or in groups, and exhibit a small angle of emission.
The German Patent Application DE 28 52 203 discloses an illumination setup for a microscope, where light from a light source is guided along optical fibres and exits the fibres at a respective end of the fibres, wherein these respective ends are arranged in an annular fashion. A further ring illumination system, based on optical fibres is for example disclosed in the German patent application DE 40 16 264.
A problem of ring illumination systems based on optical fibres is the large divergence of the light exiting an optical fibre. Likewise, annular arrangements of LEDs tend to create rather inhomogeneous illumination fields, and even if such LEDs are used in combination with state of the art collector lenses, the degree of homogeneity of the illumination field required for some applications is not achievable.
Maximum light intensity is a very important design parameter for ring lights, as these inherently are “dark field” illumination, where scattered light is often a viewing object. A main disadvantage of LEDs compared to arc lamps (a standard for bright illumination for normal microscope viewing), is that they are typically dimmer. It is often the case that LED based systems do not provide adequate light intensity at the viewed object, and that extreme care in design should be made to maximize the possible light at the image. Maximizing the light is particularly important in machine vision/inspection where integration time cannot be arbitrarily lengthened to increase the light energy available to create a well exposed image. Increase of the integration time in a machine vision system decreases the frequency of images and increases the total inspection time (image time). Since the value of an inspection machine depends on how many fields can be imaged in a fixed time, maximizing the light at the object is a very important feature.
With ring lights the area to be illuminated is typically illuminated under an oblique angle. The light beams from the individual light sources in the ring light impinge onto the surface to be illuminated in such a way that an angle between a surface normal and a central axis of a respective light beam is neither zero nor 90°. The cross section of a respective beam on the surface to be illuminated is larger than a cross section of the respective beam in the case of illumination of the surface along the surface normal. Therefore for the light beams in a ring light the intensity of the beams is distributed over a larger area on the surface than for illumination along the surface normal. The intensity of the light beams available on the surface thus is effectively reduced. In view of the above goal of maximum light intensity this is a disadvantage.